It seems there is no problem thinking about what triangles and white things are, but what of triangularity and whiteness?
This seems to be an instance of the tooth pain issue. Noun-ness was for ordinary objects (with obvious ontological status) in an earlier game, but our creativity with language led us to nounify many other words, leading to ‘objects’ with unclear ontological status.
Realism asserts that universals are real things, that triangularity does refer to something in the world, although perhaps some metaphysical realm.
Plato is an example.
Berkeley rejected Locke’s view of abstract ideas on empiricist grounds. For him triangularity is just a name that refers to all triangles, rather than referring to something in particular.
There are no abstract ideas, just particulars.
One can accept that universals refer to things in the world but be clear that they are mental things (concepts). Locke is an example.